Clear float glass
Nearly every pane in a modern building starts life right here. Here is how molten glass floating on a bath of liquid tin becomes the flat, clear sheet everything else is built from, what thicknesses you can actually order, and when plain clear glass is the right call.
What is float glass?
Clear, tinted, Low-E, tempered, laminated, whatever you end up specifying, it almost certainly began as float glass. "Float" is not a kind of glass; it is how the glass is made. And once the word clicks, it explains why modern glass is so dead flat and distortion-free when older glass never was.
Making a flat sheet used to be genuinely hard. You either blew a giant glass cylinder and sliced it open, or you cast a slab and ground and polished it smooth. Both were slow, expensive, and left the glass wavy enough to warp whatever you looked at through it. Then in the 1950s Sir Alastair Pilkington cracked the float process, and the industry never went back.
The float process
It starts with the batch: silica sand, soda ash, limestone, dolomite, and a pinch of refining agents, all fed into a furnace and melted at around 1,500°C (2,730°F). The molten glass pours out the far end as one continuous ribbon and slides straight onto a bath of molten tin.
The tin is the clever bit. A liquid always settles perfectly level, and since glass is lighter than tin, the ribbon floats right on top of it. As it drifts down the bath, gravity spreads it to a natural resting thickness of about 6–7mm and both faces come out flat, parallel, and mirror-smooth, no grinding required. That is the whole trick, and it is why float glass looks the way it does.
From the tin bath the ribbon rolls into an annealing lehr, a long oven that walks the glass down from about 600°C to room temperature nice and slow. That patience matters: rush the cooling, or let it cool unevenly, and the glass locks in internal stress that leaves it fragile and unpredictable. Cooled properly and stress-free, it earns the name annealed glass.
At the end of the line the ribbon is scored and snapped into big stock sheets and shipped off to fabricators, who handle everything after that: cutting it down, coating it, tempering it, or building it into insulating units.
Standard thicknesses
Float glass comes in a spread of thicknesses. Here are the common architectural ones, what they are called, and where they usually end up. One thing worth knowing before you order: the "nominal" thickness is just the name on the paperwork. The real thickness lands somewhere inside a tolerance band set by ASTM C1036, which is why a "6 mm" lite can legitimately measure anywhere from 5.56 to 6.20 mm.
| Nominal thickness | Actual range (ASTM C1036) | Common uses |
|---|---|---|
| 3 mm (⅛") | 2.90 – 3.20 mm | Picture frames, small panes, interior mirrors |
| 4 mm (5/32") | 3.78 – 4.19 mm | Small residential windows, storm doors |
| 5 mm (3/16") | 4.57 – 5.05 mm | Residential windows, cabinet doors |
| 6 mm (¼") | 5.56 – 6.20 mm | Most common residential and commercial; IG unit lites |
| 8 mm (5/16") | 7.42 – 8.43 mm | Larger panes, structural applications |
| 10 mm (⅜") | 9.02 – 9.53 mm | Heavy commercial, partitions, balustrades |
| 12 mm (½") | 11.91 – 12.70 mm | Structural glazing, frameless systems |
| 19 mm (¾") | 18.26 – 19.05 mm | Large structural applications, countertops |
Quality levels (ASTM C1036)
Not every sheet of clear glass is held to the same cosmetic bar. ASTM C1036, the US flat-glass standard, sorts it into three quality levels based on how many small flaws (bubbles, inclusions, scratches, distortion) are allowed to slip through:
- Type I, Quality q1 (Glazing Select): The highest quality. Minimal blemishes. Required for display cases, high-end storefronts, and wherever the glass surface is closely inspected. Also used when Low-E coatings or other surface treatments require a pristine substrate.
- Type I, Quality q2 (Glazing B): Standard quality for most architectural applications. Small blemishes are permitted within defined limits. This is the default for residential windows and commercial curtainwall units.
- Type I, Quality q3 (Greenhouse): Allows more blemishes. Used where optical quality is unimportant, greenhouse glazing, temporary enclosures, and low-visibility applications.
Optical properties of clear glass
A single 6 mm pane of clear float passes roughly 87–90% of visible light, which is about as clear as ordinary glass gets. The catch is that it lets the heat straight through too: its SHGC sits around 0.86, so most of the sun's energy, including the near-infrared you feel as warmth, comes right in. That is precisely the problem Low-E coatings were invented to fix.
Ever noticed the faint green edge on a piece of "clear" glass? That is iron (Fe₂O₃) hitching a ride in the sand it is made from, and the more iron, the greener it reads, especially edge-on or in thick pieces. Most of the time you never notice. But when the color has to be truly neutral, think art glazing, high-end retail, photo studios, you reach for low-iron glass (sold as "ultra-clear" or "optically clear"), which strips most of the iron out, pushes light transmission past 91%, and looks genuinely colorless.
Limitations of annealed glass
Plain annealed float has one serious catch, and it is all about how it breaks. Hit it hard enough and it splits into big, jagged, razor-edged shards, the dangerous "broken glass" everyone pictures. That failure mode is the whole reason the codes get fussy about where you are allowed to use it.
US building codes (IBC, IRC) demand safety glazing, meaning tempered or laminated glass, anywhere a break could put someone in the hospital:
- Doors, sidelites, and panels within 24 inches of a door
- Windows within 18 inches of the floor
- Shower and tub enclosures
- Railings and balustrades
- Skylights and overhead glazing
- Any glazing in wet or high-traffic areas
Step outside those hazard zones, though, and annealed glass is completely fine, and it is everywhere: ordinary residential windows, commercial curtainwall that sits well out of reach, and interior glazing up above head height.
When to specify clear float
Reach for plain clear annealed float when:
- The location is not a code-required safety glazing zone
- No thermal performance upgrade is needed (or it will be part of an IGU with a coated lite)
- Maximum visible light transmission is the priority
- Cost is a primary constraint
- It will be further processed (tempered, laminated, coated) before installation
Here is the thing: almost none of the glass you actually see in a finished building is "just clear float" by the time it goes in. It has been tempered, coated, laminated, or sealed into a unit somewhere along the way. But every one of those products started as this same clear annealed sheet, so nail down this one and the rest of the catalog suddenly makes a lot more sense.